Drive circuit for organic el device

ABSTRACT

It is an object of the present invention to provide a drive circuit for an organic light emitting diode which can suppress a fall in an intensity of light emission to extend the life of the organic light emitting diode. The present invention is a drive circuit for an organic light emitting diode which has plural anodes A 1  to A 3  and plural cathodes B 1  to B 4  intersecting while being opposed to each other and an organic layer having at least a light-emitting layer intervening between both the poles, sets one of both the poles as drive lines and the other of both the poles as scanning lines, and by scanning all of these scanning lines as one field at a predetermined frequency, presents light emission with the intersection portions as pixels E 11  to E 34 . Capacitors C 11  to C 34  having a predetermined added capacity, to which a bias voltage Vr in a direction of canceling charges to be charged to a parasitic capacity of the pixels E 11  to E 34  is applied, are provided.

TECHNICAL FIELD

[0001] The present invention relates to a drive circuit for an organiclight emitting diode (OLED).

BACKGROUND ART

[0002] An OLED having a light-emitting layer consisting of an organiccompound has been attracting attentions as a device realizing DC lowvoltage drive. For example, JP-A-6-32307 discloses a structure in whicha nodes consisting of a translucent film of an indium tin oxide (ITO)are formed on an upper surface of a substrate consisting of atranslucent glass, a hole injection layer, a light-emitting layer, andcathodes consisting of a film of aluminum (Al) are sequentially formedon the anode, and a power supply is connected between the anodes and thecathodes, whereby positive holes generated in the anodes are transmittedto an interface between the hole injection layer and the light-emittinglayer, where the positive holes are combined with electrons transmittedfrom the cathodes to emit a visible ray.

[0003] There are known an OLED for performing so-called passive matrixdisplay in which, in the above OLED, the anodes and the cathodes areformed in a plural strip shapes, respectively, and the anodes and thecathodes are arranged in a lattice shape, and one of the anodes and thecathodes are sequentially selected and scanned at a constant timeinterval with each intersection position of the anodes and the cathodesarranged in the lattice shape as a light emission portion (pixel) and,at the same time, the other of the anodes and the cathodes are driven bya DC constant current circuit serving as a drive source, whereby anarbitrary pixel is caused to emit light, and a driver circuit for theOLED.

[0004] Such an OLED has a problem in that, in a long-term use, chargersare accumulated in an intersection between the hole injection layer andthe light-emitting layer, whereby a light emission luminance in thepixel falls.

[0005] As a technique for solving such a problem, for example,JP-A-9-232074 discloses, as a drive circuit (method) for performing thepassive matrix display, a technique with which, at the time of switchingto the next scanning line (cathode), all the scanning lines are onceconnected to a reset voltage zero volt (or power supply voltage)consisting of the same potential for a predetermined time (reset time),whereby a parasitic capacity of the pixels, which should be caused toemit light, is charged by the drive source via drive lines (the anodes)and, at the same time, also charged by a reverse bias voltage of thescanning lines through the parasitic capacity of the pixels, which arenot caused to emit light and, consequently, since voltages at both endsof the pixels which should be caused to emit light instantly rise to apotential allowing light emission, the pixels can emit light instantly.

[0006] However, connecting all the scanning lines to the reset voltageconsisting of the same potential once at the time of switching to thenext scanning line makes an operation complex and causes enlargement andcomplication of a drive circuit. In addition, since the power supplyvoltage is applied to the unselected scanning lines, an invalid chargingcurrent, which does not contribute to light emission, flows to the drivelines via the pixels to increase a consumed current. Further, since allthe pixels including the pixels, which should be caused to emit light,are turned off during the reset time, a light emission time isrelatively reduced to cause a fall in the light emission luminance. Inorder to compensate for the fall, a peak luminance should be made higherthan usual, which facilitates a luminance fall characteristic to shortenthe life of the OLED.

[0007] In addition, for example, Japanese Patent No. 3102411 discloses atechnique with which a charging circuit for charging the parasiticcapacity (junction capacity) to a predetermined potential at the time ofrising of drive of the OLED with an output of a pulse generator isprovided in constant current drive means for driving the OLED, wherebythe junction capacity can be charged in a predetermined time (chargingtime) and, the OLED is driven without delaying the rising of a pulse, afall in the light emission luminance can be suppressed.

[0008] However, in the correction by the charging circuit, since chargesto be cancelled depend upon “current×time” or “voltage×time”, setting ofa current value, a voltage value, and a time, which are controlparameters therefor, is difficult. In addition, by providing thecharging time, since all the pixels come into a non-light emission stateduring that time, and all the pixels including the pixels which shouldbe caused to emit light are turned off, a light emission time isrelatively reduced to cause a fall in the light emission luminance. Inorder to compensate for the fall, a peak luminance should be made higherthan usual, which facilitates a luminance fall characteristic to shortenthe life of the OLED.

[0009] The present invention has been devised in view of such points,and it is an object of the present invention to provide a drive circuitwhich, through application to an OLED for performing the so-calledpassive matrix display, can extend the life of the OLED with a simpleconfiguration.

DISCLOSURE OF THE INVENTION

[0010] The present invention is a drive circuit for an organiclight-emitting diode which has anodes and cathodes opposed to each otherand an organic layer having at least a light-emitting layer interveningbetween both the poles, and presents light emission with the opposedportions as pixels, in which capacitors having a predetermined addedcapacity, to which a bias voltage is applied in a direction of cancelingcharges to be charged to a parasitic capacity of the pixels, areprovided.

[0011] In addition, the present invention is a drive circuit for anorganic light emitting diode which has plural anodes and plural cathodesintersecting while being opposed to each other and an organic layerhaving at least a light-emitting layer intervening between both thepoles, sets one of both the poles as drive lines and the other of boththe poles as scanning lines, and by scanning all of these scanning linesas one field at a predetermined frequency, while sequentially bringingany one of the scanning lines into a selected state, in synchronizationwith this, connects a drive source to the other of the scanning lines tothereby present light emission with the intersection portions as pixels,in which capacitors having a predetermined added capacity, to which abias voltage is applied in a direction of canceling charges to becharged to a parasitic capacity of the pixels, are provided.

[0012] Further, the present invention is a drive circuit for an organiclight emitting diode which has plural anodes and plural cathodesintersecting while being opposed to each other and an organic layerhaving at least a light-emitting layer intervening between both thepoles, sets one of both the poles as drive lines and the other of boththe poles as scanning lines, has a partition for insulating the scanninglines from each other with a dielectric material on at least one of boththe poles, and by scanning all of these scanning lines as one field at apredetermined frequency, while sequentially bringing any one of thescanning lines into a selected state, in synchronization with this,connects a drive source to the other of the scanning lines to therebypresent light emission with the intersection portions as pixels, inwhich capacitors having a predetermined added capacity, to which a biasvoltage is applied in a direction of canceling charges to be charged toa parasitic capacity of the pixels between a conductor film formed onthe partition and the one of both the poles, are provided.

[0013] In particular, the conductor film is connected to a bias circuitwhich is commonly connected to supply the bias voltage.

[0014] In particular, the sum of the added capacity is equal to or morethan the parasitic capacity.

[0015] In particular, the drive source performs a DC current or DCvoltage drive, and the bias voltage is a DC voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a circuit diagram of an embodiment of the presentinvention, FIG. 2 is a circuit diagram of the same, FIG. 3 is a mainpart schematic perspective view of the embodiment of the presentinvention, and FIG. 4 is a same main part schematic sectional view ofthe same.

BEST MODE FOR CARRYING OUT THE INVENTION

[0017] The present invention will be described based upon an embodimentshown in the accompanying drawings.

[0018]FIGS. 1 and 2 are a drive circuit in accordance with theembodiment and show the case in which the drive circuit has row(horizontal direction)×column (vertical direction)=3×4=12 pixels. Forexample, in the case in which such an OLED is used as a display devicefor displaying numbers of an odometer, a clock, and the like of avehicle in one row, the number of rows is determined according to thenumber of digits to be displayed, and about sixteen columns arenecessary.

[0019] Reference symbols A1 to A3 denote anodes, B1 to B4 denotecathodes, and E11 to E34 denote pixels which are located atintersections of the anodes A1 to A3 and the cathodes B1 to B4 andpresent light emission. The respective pixels E11 to E34 are representedby equivalent circuits of a light-emitting element consisting of a diodecharacteristic and a parasitic capacity connected to this in parallel.However, in the following description and the drawings, assuming thatcharacteristics of the pixels E11 to E34 are equal, the parasiticcapacity of an arbitrary pixel Exx is assumed to be Cel. Then, a drivecircuit 10 is connected to the anodes A1 to A3, and a scanning circuit20 is connected to the cathodes B1 to B4.

[0020] A drive voltage circuit 30 for outputting a drive voltage Va isconnected to the drive circuit 10. The drive circuit 10 has switches 11to 13 for selecting the anodes A1 to A3 between the drive circuit 10 andthe anodes A1 to A3 and, by turning “ON” these switches 11 to 13,connects DC constant current circuits 14 to 16 serving as drive sourcesfor drive to the anodes A1 to A3.

[0021] The scanning circuit 20 has switches 21 to 24 for sequentiallyscanning the respective cathodes B1 to B4 as scanning lines, and oneends of the respective switches 21 to 24 are connected to a reversevoltage terminal to which a cutoff voltage Vk volt supplied from a drivevoltage circuit 30 is applied and the other ends thereof are connectedto a ground terminal of zero volt, and these switches 21 to 24 arechanged over to the reverse voltage terminal (unselected) and the groundterminal (selected) in order.

[0022] Then, the drive circuit 10 and the scanning circuit 20 arecontrolled by a display control circuit 40. Note that, in the figure, inthe switches 11 to 13 of the drive circuit 10, the case in which acurrent Id is applied to any of the pixels E11 to E34 by the currentcircuits 14 to 16 is represented as “ON” and the case in which thecurrent Id is no applied is represented as “OFF”. In addition, in theswitches 21 to 24 of the scanning circuit 20, the case in which theswitches 21 to 24 are connected to an anode being scanned among theanodes B1 to B4 is represented as “selected” and the case in which theswitches 21 to 24 are connected to an anode not being scanned isrepresented as “unselected”. Consequently, only the pixels, for whichthe drive circuit 10 is “ON” and the scanning circuit 20 is “selected”,among the pixels E11 to E34 emit light.

[0023] One ends of capacitors C11 to C34 are connected to eachconnection point of the pixels E11 to E34 and the anodes A1 to A3,respectively, and the other ends of these capacitors C11 to C34 arecollectively connected to a bias circuit 50. This bias circuit 50supplies a bias voltage Vr (=Vk,>Vf) volt to the capacitors C11 to C34.Note that an arbitrary capacitor Cxx is selected and connected so as tohave an added capacity Cr equal to or more than the parasitic capacityCel of the pixel Exx to which the one end thereof is connected. However,as described above, if characteristics of the pixels E11 to E34 areequal (i.e., the parasitic capacity Cel is the same in all the pixelsE11 to E34), only there is one kind of the added capacity Cr.

[0024] Control of light emission by such a drive circuit will bedescribed. In the following description, the case in which, afterscanning the cathode B2 as a scanning line to cause the pixels E22, E32to emit light, the cathode B3 is scanned to cause the pixels E13, E33 toemit light will be described.

[0025] In order to scan the cathode B2 to cause the pixels E22, E32 toemit light, as shown in FIG. 1, the switch 22 of the scanning circuit 20is changed over to the ground terminal, which is the selected side, andthe cathode B2 is scanned. The other switches 21, 23, 24 are changedover to the reverse voltage terminal which is the unselected side.Therefore, a potential on the pixel side of the scanning circuit 20 iszero volt for the switch 22 and Vk volt for the other switches.

[0026] Current circuits 15, 16 are connected to the anodes A2, A3 by theswitches 12, 13 of the drive circuit 10, but the other switch 11 isconnected to the ground terminal. Therefore, a potential on the pixelside of the drive circuit 10 is Vf volt, which is a predeterminedpotential, for the switches 12, 13 and is zero volt for the otherswitch.

[0027] At this point, only the pixels E22, E32, for which the drivecircuit 10 is “ON” and the scanning circuit 20 is “selected”, are biasedin a forward direction, the drive current Id is flowing to the pixelsE22, E32 from the current circuits 15, 16, and a charge Qon to becharged to these pixels E22, E32 is Cel×Vf>0.

[0028] A charge Qoff1 to be charged to the other pixel E12, for whichthe scanning circuit 20 is “selected”, is zero, the charges Qon and Qoffhave a relation of Qon>Qoff, and the pixel E12 does not emit light and acurrent does not flow to the pixel E12.

[0029] A charge Qoff2 to be charged to the pixels E21, E23, E24, E31,E33, E34, for which the drive circuit 10 is “ON” and the scanningcircuit 20 is “unselected”, is Cel×(Vk−Vf)<0. That is, since Vk and Vfhave opposite polarities, a difference between them is low compared withVf, the charges Qon and Qoff2 have a relation of Qon>Qoff2, and thepixel Exx with the charge Qoff2 does not emit light and almost nocurrent flows to the pixel Exx.

[0030] A charge Qoff3 to be charged to the pixels E11, E13, E14, forwhich the drive circuit 10 is “OFF” and the scanning circuit 20 is“unselected”, is Cel×Vk<0. That is, since Vk and Vf have oppositepolarities, the charges Qon and Qoff3 have a relation of Qon>Qoff3, andthe pixel Exx with the charge Qoff3 does not emit light and almost nocurrent flows to the pixel Exx.

[0031] A charge Qc1 to be charged to the capacitors C21 to C24, C31 toC34 connected to the pixel Exx, for which the drive circuit 10 is “ON”,is Cr×(Vr−Vf)>0. This is because Vk and Vf are set to the same polarity.

[0032] A charge Qc2 to be charged to the capacitors C11 to C14 connectedto the pixel Exx, for which the drive circuit 10 is “OFF”, is Cr×Vr>0.This is because Vk and Vf are set to the same polarity.

[0033] Next, in order to scan the cathode B3 to cause the pixels E13,E33 to emit light, as shown in FIG. 2, the switch 23 of the scanningcircuit 20 is changed over to the ground terminal, which is the selectedside, and the cathode B3 is scanned. The other switches 21, 22, 24 arechanged over to the reverse voltage terminal which is the unselectedside. Therefore, a potential on the pixel side of the scanning circuit20 is zero volt for the switch 23 and Vk volt for the other switches.

[0034] Current circuits 14, 16 are connected to the anodes A1, A3 by theswitches 11, 13 of the drive circuit 10, but the other switch 12 isconnected to the ground terminal. Therefore, a potential on the pixelside of the drive circuit 10 is Vf volt, which is a predeterminedpotential, for the switches 11, 13 and is zero volt for the otherswitch.

[0035] At this point, only the pixels E13, E33, for which the drivecircuit 10 is “ON” and the scanning circuit 20 is “selected”, are biasedin a forward direction, the drive current Id is flowing to the pixelsE13, E33 from the current circuits 14, 16, and a charge Qon to becharged to these pixels E13, E33 is Cel×Vf. On the other hand, thecharge Qoff of the other pixel Exx is as described above (see Qoff1 toQoff3), the charges Qon and Qoff have a relation of Qon>Qoff, and theother pixel Exx with the charge Qoff does not emit light.

[0036] In addition, the charge Qc1 to be charged to the capacitors C1 n,C3 n (n=1 to 4, same in the following description) and the charge Qc2 ofthe capacitor C2 n are also as described above.

[0037] When shifting from FIG. 1 to FIG. 2, the charge Qoff3 charged tothe pixels E11, E13, E14 is cancelled by the charge Qc2 charged to thecapacitors C11, C13, C14 due to a relation of Qoff3=Qc2.

[0038] Similarly, the charge Qoff2 charged to the pixels E31, E33, E34is cancelled by the charge Qc1 charged to the capacitors C31, C33, C34due to a relation of Qoff2=Qc1.

[0039] On the other hand, the charge charged to the pixel E2 n and thecapacitor C2 n connected to the same is discharged because the switch 12is grounded.

[0040] Note that, since the charge Qoff1 charged to the pixel E12 iszero, there is no movement of charge in the switching from FIG. 1 toFIG. 2, and the charge Qc2 charged to the capacitor C12 connected to thepixel E12 does not change.

[0041] In addition, in the pixel E32, although the charge falls from thecharge Qon (FIG. 1) to the charge Qoff2 (FIG. 2), since the capacitorC32 connected to the pixel E32 tries to keep the charge Qc1 thereofconstant, an excess charge Qon−Qoff2 moves from the pixel E33 connectedto the ground terminal via the switch 23.

[0042] In this way, in the pixel Exx and the capacitor Cxx connected tothe pixel Exx, the charge charged to the pixel Exx (in theabove-mentioned example, pixels E11, E13, E14, E31, E33, E34) can becancelled, and deterioration due to the charge can be improved.

[0043] In addition, in the case in which the pixel Exx (in theabove-mentioned example, the pixel E32) changing from light emission tonon-light emission and the pixel Exx (in the above-mentioned example,the pixel E33) changing from non-light emission to light emission areconnected to the identical line (in the above-mentioned example, theanode A3) of the drive circuit 10, since an excessive charge thereofmoves from the former pixel to the latter pixel, injection of chargesinto the latter pixel is performed with high efficiency in a short time,rising of light emission becomes steep.

[0044] In the case in which the pixel Exx shifts from non-light emissionto light emission, that is, in a process in which a potential on thedrive circuit 10 side rises from zero volt to Vf volt, assuming that avoltage rise is ΔV, a charge not contributing to light emission of thepixel Exx=Cel×ΔV and a charge of the capacitor Cxx connected to thepixel Exx=Cr×ΔV, which are accumulated according to the parasiticcapacity Cel and the added capacity Cr, are cancelled each other becausethe voltage rise ΔV is equal.

[0045] For this purpose, the charge which is accumulated in the pixelExx and does not contribute to light emission and the charge accumulatedin the capacitor Cxx connected to the pixel Exx are required to haveopposite polarities and the latter charge is required to be set largerthan the former charge in order to completely eliminate the formercharge. Consequently, Cel≦Cr is desirable.

[0046] Note that, although the example in which the DC constant currentcircuits 14 to 16 are used as drive sources is described in FIGS. 1 and2, the present invention can be realized in the same manner as describeabove even if a DC constant voltage circuit is used. In any case, if adrive source is a DC drive source, it is desirable that the bias voltageVr is a DC voltage in order to charge the added capacity Cr of thecapacitor Cxx for canceling a charge to be charged to the parasiticcapacity Cel of the pixel Exx.

[0047] Next, a specific forming method of the capacitor Cxx will bedescribed.

[0048] As a structure of an OLED for performing so-called passive matrixdisplay, for example, JP-A-8-315981 discloses, as shown in FIGS. 3 and4, an OLED which consists of a substrate 100 having anodes A1, A2, A3 .. . Ax serving as plural first display electrodes formed on a surfacethereof, plural electrical insulating partitions 200 projecting abovethe substrate 100 which causes at least a part of the anodes A1, A2, A3. . . Ax, at least a single layer of thin film 300 of an organic ELmedium formed on the exposed parts of the anodes A1, A2, A3 . . . Ax,respectively, and cathodes B1, B2, B3, B4 . . . Bx serving as pluralsecond display electrodes formed on this thin film 300.

[0049] In such a structure, in forming the cathodes B1, B2, B3, B4 . . .Bx, a material of the cathodes B1, B2, B3, B4 . . . Bx, for example,aluminum are evaporated on the partition 200, whereby a conductor film400 consisting of the same material as the cathodes B1, B2, B3, B4 . . .Bx is also formed on the partition 200.

[0050] Here, a dielectric is used as a material forming the partition200, at least one of the anodes A1, A2, A3 . . . Ax (e.g., last Ax) isset as a connection line connected to the bias circuit 50 shown in FIGS.1 and 2 not contributing to display, and the conductor film 400 iselectrically connected to this.

[0051] More specifically, as shown in FIG. 4, in forming a through hole500 in a portion opposed to the anode Ax of each partition 200 andforming the cathodes B1, B2, B3, B4 . . . Bx and the conductor film 400on the partition 200, apart of materials thereof enters the through hole500 to electrically connect the conductor film and the anode Ax, wherebythe conductor film 400 is connected to the bias circuit 50 (see FIGS. 1and 2) which is commonly connected and supplies the bias voltage Vr.Thus, the structure can be simplified.

[0052] Consequently, the capacitor Cxx can be obtained between theconductor film 400 and the anodes A1, A2, A3 . . . Ax-1 on the partition200.

[0053] Note that the partition 200 is sandwiched by the anodes A1, A2,A3 . . . Ax in almost all the portions thereof and, for example, thecapacitor Cxx, which is obtained between a conductor film 400 a and theanodes A1, A2, A3 . . . Ax on a partition 200 a located between thecathode B1 and the cathode B2, becomes the capacitor Cxx which affectsboth a group of the pixels E11, E21, E31 . . . formed of the anodes A1,A2, A3 . . . and the cathode B1 and a group of the pixels E12, E22, E32. . . formed of the anodes A1, A2, A3 . . . and the cathode B2.

[0054] Therefore, in the case in which such a structure is adopted, astructure in which the pixel Exx and the capacitor Cxx corresponds oneto one as shown in FIGS. 1 and 2 is not realized. Consequently, it isnecessary to set a sum of the added capacity Cr of the capacitor Cxx tobe equal to or more than a sum of the parasitic capacity Cel for theExx, and more preferably to be relatively larger taking into account thenumber of pixels Exx which are affected by the capacitor Cxx.

[0055] The present invention can be realized not only in theabove-mentioned OLED for performing so-called passive matrix display butalso in an OLED for performing segment display as long as the presentinvention can be applied to the OLED in the same manner and the OLED hasanodes and cathodes opposed to each other and an organic layer having atleast light-emitting layer intervening between both the poles, andpresents light emission with the opposed portions as pixels by providinga capacitor having a predetermined added capacity to which a biasvoltage in a direction of canceling charges to be charged to a parasiticcapacity of the pixels.

INDUSTRIAL APPLICABILITY

[0056] As described above, in the present invention, reduction of alight emission time according to setting of a reset time or a chargingtime required by the related art is eliminated, and a light emissionpeak luminance falls. In addition, a light emission risingcharacteristic is improved to have better linearity for gradation, andmarketability is improved. Further, it becomes unnecessary to add aspecial timing circuit to a drive sequence, and a drive circuit can bereduced in size and simplified. Thus, provision of a drive circuit whichcan extend the life of the OLED with a simple structure can be realized.

1. A drive circuit for an organic light-emitting diode which has anodesand cathodes opposed to each other and an organic layer having at leasta light-emitting layer intervening between both the poles, and presentslight emission with the opposed portions as pixels, characterized inthat capacitors having a predetermined added capacity, to which a biasvoltage is applied in a direction of canceling charges to be charged toa parasitic capacity of the pixels, are provided.
 2. A drive circuit foran organic light emitting diode which has plural anodes and pluralcathodes intersecting while being opposed to each other and an organiclayer having at least a light-emitting layer intervening between boththe poles, sets one of both the poles as drive lines and the other ofboth the poles as scanning lines, and by scanning all of these scanninglines as one field at a predetermined frequency, while sequentiallybringing any one of the scanning lines into a selected state, insynchronization with this, connects a drive source to the other of thescanning lines to thereby present light emission with the intersectionportions as pixels, characterized in that capacitors having apredetermined added capacity, to which a bias voltage is applied in adirection of canceling charges to be charged to a parasitic capacity ofthe pixels, are provided.
 3. A drive circuit for an organic lightemitting diode which has plural anodes and plural cathodes intersectingwhile being opposed to each other and an organic layer having at least alight-emitting layer intervening between both the poles, sets one ofboth the poles as drive lines and the other of both the poles asscanning lines, has a partition for insulating the scanning lines fromeach other with a dielectric material on at least one of both the poles,and by scanning all of these scanning lines as one field at apredetermined frequency, while sequentially bringing any one of thescanning lines into a selected state, in synchronization with this,connects a drive source to the other of the scanning lines to therebypresent light emission with the intersection portions as pixels,characterized in that capacitors having a predetermined added capacity,to which a bias voltage is applied in a direction of canceling chargesto be charged to a parasitic capacity of the pixels between a conductorfilm formed on the partition and the one of both the poles, areprovided.
 4. A drive circuit for an organic light emitting diodeaccording to claim 3, characterized in that the conductor film isconnected to a bias circuit which is commonly connected to supply thebias voltage.
 5. A drive circuit for an organic light emitting diodeaccording to any one of claims 1 to 3, characterized in that the sum ofthe added capacity is equal to or more than the parasitic capacity.
 6. Adrive circuit for an organic light emitting diode according to any oneof claims 1 to 3, characterized in that the drive source performs a DCcurrent or DC voltage drive, and the bias voltage is a DC voltage.